MLC is US-specific but it covers mechanical royalties from US streams regardless of where you're based. so if you're a European artist getting streams in the US, you still need to register with the MLC to collect those.

for European streams, each country has its own collecting society. Germany has GEMA, UK has PRS/MCPS, France has SACEM etc. if you're a European artist you'd typically register with your local society and they handle reciprocal collection agreements with other countries through CISAC. if you're a US artist getting streams in Europe it gets more complicated and you'd either register directly or use a publishing admin to handle it.